1. Field of the Invention
The present invention relates to ophthalmic lens processing and, more particularly, to lens blocks for mounting ophthalmic edged lens blanks.
2. Background Information
Eyeglasses commonly utilize lenses having convex outer surfaces and concave inner surfaces. The two surfaces of each lens have different curvatures to obtain the desired optical refraction for the lens. For many lenses, the inner surface is in the shape of a section of a torus. The direction along which the longer radius is generated is referred to as the cylinder axis of the lens. These lenses are produced from a circular glass or plastic lens blank which is subjected to multiple stages of cutting and polishing on the surfaces. A lens holding chuck, or lens block, is adhered to the convex outer surface of the circular lens blank, otherwise known as blocking by techniques known in the art, to prepare the lens blank for surfacing. See for example, FIG. 1 of U.S. Pat. No. 5,520,568 and the disclosure associated with that figure (particularly, columns 1-3), which are incorporated herein by reference in their entirety.
The process of adhering the convex outer surface of the lens blank to the lens block generally involves placing tape, such as the tape described in U.S. Pat. No. 4,287,013, to said outer surface, e.g., the finished face of a semi-finished lens blank. The lens/tape subassembly is attached to the block by using a low melting temperature alloy. For example, one typical alloy having a melting point of approximately 117.degree. F. (47.2.degree. C.) and comprising 45% bismuth, 23% lead, 8% tin, 5% cadmium and 19% indium can be used. That alloy is injected in liquid form between the block and the tape and conforms to the convex face of the lens blank.
Presently, it is common to maintain the mounted lens blank on the lens block throughout the surfacing process to avoid the remounting of the lens blank on the lens block.
Earlier lens shaping techniques would remove and remount the lens blank on the lens block at various stages of surface processing. A principal problem in reattaching the lens blank to the lens block is the proper alignment of the optical center of the lens blank with the center point and cylinder axis of the lens block. See, for example, the lens blocks shown in U.S. Pat. Nos. 2,352,616; 2,545,447; 3,015,196; 3,049,766; 3,192,676; and 4,149,344. Following the surfacing of the lens blank, the lens/tape subassembly is removed from the lens block by methods known in the art and the protective tape removed, usually by manually peeling it off the surface. Thereafter, the peripheral edge of the circular blank is cut to the final frame or lens shape, which is also known as edging. In this step, the lens is attached to an edging block by holding mechanisms known in the art, e.g., U.S. Reissue Pat. No. 31,897, and the peripheral edge machined to the desired shape for the selected frame.
The use of a low melt temperature alloy to attach the lens blank/protective tape subassembly to the lens block suffers from certain drawbacks. For example, the hot alloy can damage the lens, e.g., by cracking a glass lens or warping a plastic lens. It can also create thermal patterns on the convex outer surface of a plastic lens, which unless removed, are seen by the lens wearer as patterns of distortion. Generally, the adhesive or low melting temperature attachment alloy utilized to attach the lens block to the lens blank is carefully selected to minimize the foregoing drawbacks. Certain prior art lens blocks have caused uneven heating of a plastic lens blank, thereby resulting in the generation of thermal patterns in the lens. When the thermal patterns are near the edge, they can be removed from circular lens blanks in the subsequent edging operation. However, if the marking extends too far into the center of the circular lens blank, the thermal patterns cannot be removed in the edging operation.
Laminated lenses which include at least two layers of glass or plastic to form the lens have recently been described. The likelihood of having the distortion problem discussed above will increase in the case of a laminated lens, particularly where the convex lens section adjacent to the lens block is thin. Moreover, there is a tendency for the center of the laminated lens to be heated higher than the edges by the attachment alloy. Recently, electro-optical lenses such as electrochromic lenses have been described. These lenses may be a laminated lens assembly which includes an electrically activated section of one or more electrically activated layers between the outer lens layers. Each electrically activated layer is positioned between two electrically conductive layers, e.g., electroconductive metal oxide films. In the construction of the electro-optical lens, the conductive layers are insulated from each other to prevent a short circuit, and each conductive layer generally is provided with a separate lead or contact point for connection to the controlling electrical circuit. These leads or contact points can only be easily attached to the conductive layers after the lens has been edged to its final shape for the selected frame. Alternatively, direct contact to a bus bar (i.e., without leads) is possible.
In the case of an electrochromic laminated lens, surfacing of the lens typically occurs after the lens is assembled and the leads or contact points attached. Conventional lens blocks used with conventional lens blanks cannot be used with an edged non-surfaced electrochromic laminate lens, because the support area which has the greatest thermal gradient is generally within the vision area.
It is an object of the present invention to provide an efficient method for surfacing edged lens blanks by providing an improved lens block in which an edged lens blank may be readily and easily attached thereto in an appropriate aligned position. A further object of the present invention is to provide a lens block which would provide a sufficiently large contact area between an attachment material (e.g., a metal alloy, wax or thermoplastic organic material having an appropriate melting point) and the front surface of the edged lens blank to avoid patterns of thermal distortion. Yet another object of the present invention is to provide a lens block which is easy and economical to manufacture and use.